As illustrated in FIG. 3, a conventional capacitive pressure sensor has: a detecting capacitor formed including a diaphragm, which is deformed when applied with pressure, and a fixed electrode; and a fixed capacitor connected in series with the detecting capacitor, and by applying a square wave voltage to these capacitors, detects a partial voltage applied to the detecting capacitor. By detecting the partial voltage applied to the detecting capacitor as described, a pressure applied to the diaphragm can be measured.
Specifically, the capacitive pressure sensor is configured to detect the partial voltage applied to the detecting capacitor with an operational amplifier (first stage amplifier), synthesize a DC voltage from an output voltage of the first stage amplifier using an inverting and non-inverting circuit (including analog switches), and operate the pressure on the basis of the magnitude of the synthesized DC voltage.
However, a shift in phase or potential occurs in the inverting and non-inverting circuit due to the effect of an environment or a variation in power supply voltage, and consequently a measurement error occurs.
Also, the synthesized DC voltage includes a noise component due to switching of the inverting and non-inverting circuit, and consequently a measurement error occurs.
Further, in the case where the first stage amplifier has an offset error caused by its temperature characteristics, a drift, or the like, the offset error gives rise to a measurement error.
In addition, noise occurring in the first stage amplifier or the inverting and non-inverting circuit directly becomes a noise component, and consequently a measurement error occurs.
Also, as disclosed in Patent Literature 1, there has been considered a capacitive pressure sensor that inputs a partial voltage of a detecting capacitor and a partial voltage of a fixed capacitor (reference capacitor) to an operational amplifier (differential amplifying means).
However, the capacitive pressure sensor is one that outputs an output voltage outputted from the operational amplifier through a bandpass filter and a detector circuit, and how voltage (signal) outputted from the detector circuit is treated is unclear.